The critical effect of electron acceleration under enhanced electric field near cathode on the formation of runaway electrons and diffuse discharge in atmosphere

Abstract

Runaway electrons (RAEs) are believed to affect the dynamics of ultra-fast gas breakdown significantly. In this work, considering the field enhancement effect near the micro-protrusion on the cathode surface, the formation of RAEs and diffuse discharge in atmospheric pressure air is investigated by two-dimensional particle-in-cell/Monte Carlo collision simulation. It is found that the beam amplitude of RAEs is dictated by the field enhancement factor and the initial energy of electrons obtained near the micro-protrusion is decisive for their converting to RAEs, which precede the low energy electrons and guide the discharge propagation by improving pre-ionization. As a result, the discharge transfers from the filamentary mode without RAEs to the diffuse mode under the high pre-ionization degree due to RAEs and a wide streamer with a diameter comparable with the gap distance is formed, which transfers from spherical to conical shape. The results of this study illustrate the fundamental process of RAE formation and how RAEs influence streamer dynamics during ultra-fast gas breakdown process.